Integrated Watershed Management
What is a watershed?
A watershed is a basin-like landform defined by highpoints and ridgelines that descend into lower elevations and stream valleys.
A watershed carries water “shed” from the land after rain falls and snow melts. Drop by drop, water is channeled into soils, ground waters, creeks, and streams, making its way to larger rivers and eventually the sea.
The important thing about watersheds is: what we do on the land affects water quality for all communities living downstream.
CHARACTERISTICS OF WATERSHEDS
- SIZE: It helps in computing parameters like precipitation received, retained, drained off.
- SHAPE: Different shapes based on morphological parameters like geology and structure
- PHISIOGRAPHY: Lands altitude and physical disposition.
- SLOPE: It controls the rainfall distribution and movement:
- CLIMATE: It decides the quantitative approach.
- DRAINAGE: It determines the flow characteristics and so the erosion behavior.
- VEGETATION: Information of species gives a sure ground for selection plants and crops.
- GEOLOGY AND SOILS: Their nature determines size, shape, physiographic, drainage and groundwater conditions. Soils, derivative of rocks are the basic to greenery
- HYDROLOGY: Basic to final goal of growing greenery in a watershed. It helps in quantification of water available.
- HYDROGEOLOGY: Availability of groundwater.
- SOCIOECONOMICS: Statistics on people and their health, hygiene, wants and wishes are important in managing water.
- “A watershed is a highly evolved series of processes that convey, store, distribute, and filter water that, in turn, sustain terrestrial and aquatic life”.
- It is therefore essential to understand how trees and wetlands, floodplains and uplands are related to water cycle and management needed for its sustenance
Understanding water cycle
- Only 30-40 percent of the rain or snow that fall on the ground goes directly to streams. Most of it, is taken up and used by plants.
- Some water penetrates soils and moves below as groundwater, feeding forests and replenishing aquifers.
- After infiltrating natural systems, water evaporates from rivers and wetlands, soils and plants.
- It returns to the atmosphere to fall again as precipitation.
Water cycling cools the planet, cleans the air, and sustains life.
Human impacts on water cycle
- Human use chemicals for maintaining lawn, agricultural fertilizers, salt on roads in winter, oils from exhaust on highways, discharge of chemicals from factories, affects water quality downstream.
- In the built environment, when vegetation is removed, the nature’s mechanisms for storing and cleaning water is reduced.
- Rain is piped away before soils can retain it, increasing the likelihood of flooding and erosion.
- “Watershed management is the study of the relevant characteristics of a watershed aimed at the sustainable distribution of its resources and the process of creating and implementing plans, programs, and projects to sustain and enhance watershed functions that affect the plant, animal and human communities within a watershed boundary”.
- Features of a watershed that agencies seek to manage include water supply, water quality, drainage, storm water runoff, water rights, and the overall planning and utilization of watersheds” (Wikipedia)
Why Watershed management?
- It helps to understand and implement management of land, water, energy and greenery integrating all the relevant approaches appropriate to socioeconomic background for a sustainable development of a watershed
- Greening of the watershed through proper management of land water and energy resource.
The objectives of watershed management
- Conserving soil and water
- Improving the ability of land to hold water
- Rainwater harvesting and recharging
- Growing greenery – trees, crops and grasses
Factors influencing precipitation (dynamics of hydrological cycle)
- Area Rock Type Drainage Density
- Shape Soil Precipitation / Temp
- Slope Land Use Tidal Conditions
- Large basins receive more precipitation than small therefore have larger runoff
- Larger size means longer lag time as water has a longer distance to travel to reach the trunk river
- Elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size
- Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
- Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb
- Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers
- The more infiltration occurs the longer the lag time and shallower the rising limb
- Urbanization – concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag
- Afforestation – intercepts the precipitation, creating a shallow rising limb and lengthening the time lag
- A higher density will allow rapid overland flow
Precipitation & Temperature
- Short intense rainstorms can produce rapid overland flow and steep rising limb
- If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off
- Snow on the ground can act as a store producing a long lag time and shallow rising limb. Once a thaw sets in the rising limb will become steep
A Drainage Basin
A drainage divide separates the drainage basin and its watershed from other basins
How do we study a Watershed?
If sampling site is at the red dot on the stream, the water to be tested could have interacted with any of the watershed area upstream of that point.